In simple terms, a generator is a device for converting mechanical energy into electrical energy and works by electromagnetic induction. A power source drives a coil winding, causing it to rotate between the poles of a permanent magnet or electromagnet. As the coil winding spins and cuts through the lines of force between the poles of the magnet, potential energy and electric current is generated and flows through the coil winding. The electric current that is generated may be either direct current (DC) or alternating current (AC). In AC generation, a sinusoidal output waveform is produced; no energy is induced as the coil winding rotates parallel to the magnetic flux lines, while maximum power is achieved when the coil winding is rotating tangential to the magnetic flux lines.
The first electric generators, or dynamos, were modeled and built in the 1830s. By the end of the nineteenth century, significant advances were being made in the field of electrical generation, particularly by Nikola Tesla. In 1890, Tesla disclosed a pyromagneto-electric generator in U.S. Pat. No. 428,057, in which he recognized that the magnetic properties of iron and other magnetic substances may be compromised by raising the material to a certain temperature and restored by again lowering the temperature. Also in 1890, Tesla disclosed an electrical transformer or induction device in U.S. Pat. No. 433,702.
Alternating current generators in use at the time typically provided from one to three hundred alterations of current per second. It was soon recognized that higher rates of alteration would be an advantage. Producing higher rates of alteration with generator designs at the time, however, was difficult and resulted in decreased efficiency, primarily due to high magnetic leakage, and improved generator designs were sought. In U.S. Pat. No. 447,921, Tesla discloses a field-magnet core made up of two independent parts formed with grooves for the reception of one or more energizing coils. The energizing coils are completely surrounded by the iron core, except on one side, where there is a narrow opening between the polar faces of the core, and the polar faces of the core are formed with many projections or serrations. This field-magnet design produced less magnetic leakage but still did not operate at a desired level of efficiency.
In 1894, Tesla disclosed an electric generator in U.S. Pat. No. 511,916. This generator was capable of continued production of electric currents of constant period by imparting the movements of a piston to a core or coil in a magnetic field.
By the twentieth century, more reliable turbines were in use, capable of providing 50-60 Hertz power with 3000-3600 alternations of current per second. In U.S. Pat. No. 1,061,206, Tesla discloses a turbine that improves the use of fluids as motive agents by causing a propelling fluid to move in natural paths or stream lines of least resistance, avoiding losses due to sudden variations while the fluid is imparting energy. This method, when coupled with power generating equipment, provided a more efficient and reliable means of hydraulic power synthesis.
Another conventional generator example is the Detroit Edison generator. The Detroit Edison generator includes an outer extruded stationary permanent magnet with opposite magnetic poles forming an air gap at the center, with a number of windings rotated within the air gap to induce current in the rotating windings. As with other early generator designs, increased and improved efficiency was sought, often realized by increasing the length of the cylindrical generator.
Generator designs continued to advance in the twentieth century, where improvements made to the above-identified generator designs frequently focused on improving efficiency. U.S. Pat. No. 3,538,364, to Favereau, discloses a rotary electric machine comprising a fixed primary stator in the form of a pair of concentrically arranged inner and outer stator elements having magnetic poles and between which, in an air gap, the secondary cylindrical rotor having a winding thereon is mounted for rotation. The magnetic stator provides a 360-degree air gap between opposite magnetic poles in the inner and outer stator. This arrangement reduced the size of leakage fluxes and reduced the volume of the coils situated around the poles, permitting increases in the working induction in the cylindrical air gap.
More recently, improvements have recognized and addressed optimizing the waveshape of the generator output to maximize generator output and improve efficiency. In U.S. Pat. No. 5,650,680, Chula discloses a permanent magnet generator having a rotor including a plurality of permanent magnets generating an operative magnetic flux field, seeking to create an output voltage signal with reduced harmonic content.
Conventional generator designs typically include contacts, or “brushes,” that rotate relative to electrical contacts and provide a circuit for electricity to flow through. Brushes, however, require regular maintenance and replacement as they become worn. Additionally, the electrical resistance of the brushes and the mechanical frictional loss between the brushes and the contacts decrease generator efficiency. These drawbacks were recognized by Rakestraw et al. in U.S. Pat. No. 5,696,419, which discloses an electrical generator with a plurality of C-shaped stator members made of magnetically permeable material. A flat ring-shaped rotor defines a periphery, and a plurality of permanent magnets are positioned around the periphery. The rotor is positioned with the magnets of the rotor disposed in the gap defined by the stator members, so that when the rotor is rotated by a prime mover to move the magnets through the gap, an electrical current is induced in the stator windings.
Others have sought to improve generator efficiency by not only eliminating brushes but also improving per-magnet rotor excitation. In U.S. Pat. No. 6,462,449, Lucidarme et al. disclose a rotating electric machine where the rotor includes a magnetic field core provided with radial teeth, uniformly distributed at its periphery. Annular magnets are arranged on either side of the core axial ends and magnetic end flanges pressing the annular magnets against the core. Magnetic bars link the end between each of the bars and at least the side walls of the core radial teeth defining the spaces. The stator includes a magnetic core, excitation coils arranged on either side of the core, a stator coil wound on the core, and a magnetic ring in contact with the core and provided with radial rims cooperating with the axial rims of the rotor end flanges to form paths for the return flux.
While generator efficiencies have been increased through mechanical and electrical engineering methods as described above, there is still room for significant advancement and improvement. Relatively recent advancements in modern materials science have been applied to generator design and manufacture. For example, superconductive materials have been used in the construction of generator components. These materials provide a reduced resistance to the flow of electricity, and when used in generator components, superconductive materials have been shown to increase overall efficiency on the order of approximately 1%-3% in some applications, a relatively small gain that is quickly appreciated in large-scale generators.
An example of a trapped-field superconducting generator is disclosed in U.S. Pat. No. 5,325,002, to Rabinowitz et al. This motor/generator includes superconductive material in either the stator or the rotor and a magnetic field generator is included in the other of these two members. Induced fields in a torque-shield provide coupling between the stator and the rotor during the start-up phase of the motor/generator, and then a trapped field in the superconductor provides coupling between the stator and rotor thereafter.
U.S. Pat. No. 6,169,352, to Hull, discloses another example of a trapped-field superconducting motor generator. The motor generator includes a high temperature superconductor rotor and an internally disposed coil assembly. The motor generator superconductor rotor is constructed of a plurality of superconductor elements magnetized to produce a dipole field. The coil assembly can be either a conventional conductor or a high temperature superconductor. The superconductor rotor elements include a magnetization direction and c-axis for the crystals of the elements and which is oriented along the magnetization direction.
The above-identified generators are typically bulky and complex. Further, their designs are static, producing a single output and requiring manufacturing retooling to produce an alternate output form. Additionally, even in conventional superconducting generators, increased efficiency is desired, particularly by large-scale generation facilities and public utilities, where a small increase in efficiency can lead to a significant cost savings over time.
Therefore, there is a need for a cost-effective generator that operates with increased efficiency. There is also a need for a generator that is reconfigurable, capable of producing multiple desired outputs from a single tooled design. There is also a need for a generator that is capable of producing outputs over longer periods of time, with greater reliability and reduced maintenance costs. Further, there is a need for a generator that is adaptable to a greater range of inputs, i.e. combustion, nuclear, wind, water, and other renewable energy resources.